Huntington's disease (HD) is a neurodegenerative disorder caused by the mutation of the huntingtin gene. Alteration of this widely expressed single gene results in a progressive, neurodegenerative disorder with a large number of characteristic symptoms. Huntington's disease is an autosomal dominant disorder, with an onset generally in mid-life, although cases of onset from childhood to over 70 years of age have been documented. An earlier age of onset is associated with paternal inheritance, with 70% of juvenile cases being inherited through the father. Symptoms have an emotional, motor and cognitive component. Chorea is a characteristic feature of the motor disorder and is defined as excessive spontaneous movements which are irregularly timed, randomly distributed and abrupt. It can vary from being barely perceptible to severe. Other frequently observed abnormalities include dystonia, rigidity, bradykinesia, ocularmotor dysfunction and tremor. Voluntary movement disorders include fine motor incoordination, dysathria, and dysphagia. Emotional disorders commonly include depression and irritability, and cognitive component comprises subcortical dementia (Mangiarini et al., 1996. Cell 87:493-506). Changes in HD brains are widespread and include neuronal loss and gliosis, particularly in the cortex and striatum (Vonsattel and DiFiglia. 1998. J. Neuropathol. Exp. Neurol., 57:369-384).
The HD mutation is a CAG expansion that results in the expansion of a poly-glutamine tract in the huntingtin protein, a 350 kDa protein of unknown function (Huntington Disease Collaborative Research Group, 1993. Cell. 72:971-83). The normal and expanded HD allele size have been found to be CAG6-37 and CAG35-121 repeats, respectively. Longer repeat sequences are associated with earlier disease onset. The mechanism by which the expansion results in pathology is unknown. However, the absence of an HD phenotype in individuals deleted for one copy of huntingtin, or increased severity of disease in those homozygous for the expansion suggests that the mutation does not result in a loss of function (Trottier et al., 1995, Nature Med., 10:104-110). Transcriptional deregulation and loss of function of transcriptional coactivator proteins have been implicated in HD pathogenesis. Mutant huntingtin has been shown specifically to disrupt activator-dependent transcription in the early stages of HD pathogenesis (Dunah et al., 2002. Science 296:2238-2243). Gene profiling of human blood has identified 322 mRNAs that show significantly altered expression in HD blood samples as compared to normal or presymptomatic individuals. Expression of marker genes was similarly substantially altered in post-mortem brain samples from HD caudate, suggesting that upregulation of genes in blood samples reflects disease mechanisms found in brain. Monitoring of gene expression may provide a sensitive and quantitative method to monitor disease progression, especially in the early stages of disease in both animal models and human patients (Borovecki et al., 2005, Proc. Natl. Acad. Sci. USA 102:11023-11028).
Identification of the gene has allowed for the development of animal models of the disease, including transgenic mice carrying mutated human or mouse forms of the gene. Models include mice carrying a fragment of the human gene, typically the first one or two exons, which contains the glutamine expansion, in addition to the undisrupted wild-type, endogenous, mouse gene; mice carrying the full length human huntingtin with an expanded glutamine repeat region, again with the endogenous mouse gene; and mice with pathogenic CAG repeats inserted into the CAG repeat region. All of the models have at least some shared features with the human disease. These mice have allowed for the testing of a number of different therapeutic agents for the prevention, amelioration and treatment of HD (see, e.g., Hersch and Ferrante, 2004. NeuroRx. 1:298-306) using a number of endpoints. The compounds are believed to function by a number of different mechanisms including transcription inhibition, caspace inhibition, histone deacetylase inhibition, antioxidant, huntingtin inhibition/antioxidant biogenergetic/antioxidant, antiexcitotoxic, and antiapoptotic.
A number of authors have reported that the repression of the mutant huntingtin transgene in animal models of HD reduces the symptoms associated with the disease, (see e.g. Diaz-Hernandez et al., (2005. J. Neurosci. 25:9773-81; incorporated herein by reference). Wang et al., (2005. Nuerosci. Res. 53:241-9; incorporated herein by reference) report that small interfering RNAs (siRNAs) directed against the huntingtin gene in the mouse model R6/2 inhibited transgenic huntingtin expression and significantly prolonged longevity, improved motor function and slowed loss of body weight.
Machida et al., (2006. Biochem. Biophys. Res. Commun. 343:190-7; incorporated herein by reference), report that recombinant adeno-associated virus (rAAV)-mediated delivery of RNA interference (RNAi) into the striatum of a HD mouse model ameliorated neuropathological abnormalities associated with HD, such as insoluable protein accumulation and down-regulation of DARPP-32 expression. Importantly, the authors state that neuronal aggregates in the striatum were reduced after RNAi transduction in the animals compared to those at the time point of RNAi transduction.
Harper et al., (2005. PNAS 102:5820-25; incorporated herein by reference), found that RNAi directed to huntingtin reduced huntingtin mRNA and protein expression in cell culture and a HD mouse model. The authors report that huntingtin gene silencing improved behavioral and neuropathological abnormalities associated with HD.
Rodrigues-Lebron et al., (2005. Mol. Ther. 12:618-33; incorporated herein by reference), report that a recombinant adeno-associated viral serotype 5 (rAAV5) gene transfer of RNAi to suppress the levels of striatal mutant huntingtin in the R6/1 HD transgenic mouse resulted in reduced levels of huntingtin mRNA and protein. The reduction in huntingtin was concomitant with a reduction in the size and number of neuronal intranuclear inclusions and other markers of HD, and resulted in delayed onset of the rear paw clasping phenotype exhibited by the R6/1 mice.
Nguyen et al., (2005. PNAS, 102:11840-45; incorporated herein by reference), used the metal-binding compound clioquinol to treat PC12 cells expressing the mutant huntingtin gene and found reduced accumulation of mutant protein. Treating the HD mouse model R6/2 with clioquinol resulted in improved behavioral and pathologic phenotypes, including decreased huntingtin aggregate accumulation, decreased striatal atrophy, improved rotarod performance, reduction of weight loss, normalization of blood glucose and insulin levels, and extension of lifespan, supporting the conclusion that reduction in mutant huntingtin protein is therapeutic for HD.
Based on these and other studies, one of skill in the art recognizes that reducing the expression of the mutant huntingtin gene will be therapeutic for HD.